FIG. 1 is a block diagram of a conventional servo control system of an optical drive. The servo control system comprises an optical pickup 11, a spindle motor 13, an optical pickup driving unit 15, a spindle motor driving unit 17, and a servo control unit 19.
Firstly, according to an optical signal derived from the optical pickup 11, the servo control unit 19 can control the optical pickup 11 to process a focusing, a track jumping, a tracking on, or a track following actions by using the optical pickup driving unit 15. Moreover, the servo control unit 19 can also control the spindle motor 13 by using the spindle motor driving unit 17. The spindle motor 13 can derive a frequency generator signal (FG signal) to the servo control unit 19 according to the rotating speed of itself. After the FG signal is received by the servo control unit 19, the rotating speed of the spindle motor 13 is detected, and the servo control unit 19 can further control the rotating speed of the spindle motor 13 by using the spindle motor driving unit 17.
FIG. 2 is a diagram showing an ideal optical disc track and an actual optical disc track on an optical disc. In FIG. 2, the circle track 20 stands for an ideal optical disc track, and the elliptic track 21 stands for an actual optical disc track due to the deviation of disc manufacturing procedure, wherein the difference between the ideal optical disc track 20 and the actual optical disc track 21 is defined as a run-out error. For making the optical pickup 11 capable of precisely processing the tracking on or the tracking following, the run-out error must be accurately measured and compensated by the optical servo control system. In another words, after the optical disc is loaded into the optical drive but before the optical disc is accessed, the run-out error must be accurately measured by the optical servo control system, therefore, the servo control unit 19 can provide a specific offset to the optical pickup 11 for compensating the run-out error. Because the proper offset is continuously adjusted and then is provided to the optical pickup 11 during the data reading or data writing process of the optical drive, the tracking on and tracking following can be precisely processed no matter the optical drive is processed at a higher or a lower CAV (constant angular velocity).
For example, three Hall sensors (U, V, W) are implemented in the spindle motor 13 and Hall sensors are mounted around the spindle motor 120 from each other. A FG signal as depicted in FIG. 3A is resulted in when the three Hall sensors (U, V, W) are sensing the rotating position of the optical disc while the spindle motor 13 is rotating. And then, the FG signal is generated for the servo control unit 19 to executing a position learning procedure on the optical disc. In another words, there will be three pulses (pulse U, V, and W) occurred in the FG signal after the spindle motor 13 rotates one revolution. Moreover, a circuit implemented in the spindle motor 13 can also generate a spindle motor synchronous signal according to the FG signal. As depicted in FIG. 3B, there are three revolutions (revolution 0, 1, and 2) occurred in the spindle motor synchronous signal which is corresponding to the FG signal depicted in FIG. 3A. FIG. 3C is a diagram showing the relationship between 3 fixed positions on the optical disc and the U, V, W pulses of FG signal. The servo control unit 19 can use the three pulses occurred in the FG signal to excuse the position learning procedure of the optical disc. In another words, the three pulses occurred in the FG signal can determine three fixed positions on the optical disc.
Generally speaking, more fixed positions on the optical disc can make the optical disc more precisely controlled. Thus, the conventional optical disc servo control system multiplies the frequency of the FG signal by a frequency multiplier technique, and the frequency-multiplied FG signal includes more pulses in one revolution. In other words, the more pulses in one revolution the more fixed positions on the optical disc. Generally, the frequency ratio of the original FG signal to the frequency-multiplied FG signal (frequency-multiplier value) is set between 1˜64 according to the specific requirements. FIG. 4A is a diagram showing a frequency-multiplied FG signal having a frequency-multiplier value equal to 3. As depicted in FIG. 4A, there are nine pulses (pulse 0, 1, 2, 3, 4, 5, 6, 7, and 8) occurred in the frequency-multiplied FG signal after the spindle motor 13 rotating one revolution. FIG. 4B is a diagram showing the relationship between the nine fixed positions and corresponding pulses on an optical disc. By receiving the frequency-multiplied FG signal, the servo control unit 19 can determine more fixed positions on the optical disc, and provides a proper offset to the optical pickup 11 for compensating the run-out error precisely.
For generating the frequency-multiplied FG signal, a high-frequency reference clock signal is necessarily involved, and to be compared with the FG signal. However, if the frequency ratio of the reference clock signal to the FG signal is not an integer, each pulse occurred in the frequency-multiplied FG signal may have different period, so as the optical drive may not able to precisely compensate the run-out error.
Actually, the period of each single pulse occurred in the frequency-multiplied FG signal is given by the period of the FG signal divided by the frequency-multiplier value. To implement by a digital circuit, it needs a counter to count the reference clock signal to obtain the period of the frequency-multiplied FG signal. For example, assuming a frequency-multiplied FG signal has a frequency-multiplier value equal to 3; the time for the spindle motor 13 rotating one revolution is Trev; and there are 22 clocks occurred in the reference clock signal when the spindle motor 13 rotating one revolution, and the period of each clock is t. Ideally, each of the nine pulses occurred in the frequency-multiplied FG has a period of
            22      9        ⁢    t    ,      or    ⁢                  ⁢    2    ⁢          4      9        ⁢    t    ,as depicted in FIG. 5A. However, it is hard to generate a signal does not have an integer ratio to the period of the reference clock signal by a digital circuit. Therefore, the conventional frequency-multiplier FG generating circuit may make each pulse have a period 2t by ignoring the remainder, as depicted in FIG. 5B. However, a truncation error is resulted in if the remainder is ignored, and the time for the frequency-multiplied FG signal having nine pulses is less than the Trev, which is a time for the spindle motor 13 to rotate one revolution. Or, the conventional frequency-multiplier FG generating circuit may randomly add the remainder to the nine pulses having an integer period. As depicted in FIG. 5C, the remainder 4 is respectively added to the first four pulses to make each of the first four pulses has a period 3t and each of the last five pulses has a period 2t. Therefore, the time for the frequency-multiplied FG signal having the nine pulses is the same as the Trev. However, the first four pulses each having a period 3t and the last five pulses each having 2t also result in another situation that the triggering time points of the pulses occurred in the actual frequency-multiplied FG signal differ from the fixed positions of the optical disc. In some worst cases, the positing error between the triggering time points of the pulses occurred in the actual frequency-multiplied FG signal and the pulses occurred in the ideal frequency-multiplied FG signal is greater than it which is beyond the ability of the optical drive compensating the run-out error efficiently.
FIG. 6 is a chart showing the comparing relationship between the actual frequency-multiplied FG signal (frequency-multiplier value=3) and the ideal frequency-multiplied FG signal (frequency-multiplier value=3). As depicted in FIG. 6, each pulse in the ideal frequency-multiplied FG signal has a period
            22      9        ⁢    t    ;however, the period of the pulse occurred in the actual frequency-multiplier FG signal is 2t or 3t. In some cases, the positioning error between the ideal frequency-multiplied FG signal and the actual frequency-multiplied FG signal is worst to
          ⁢      2    ⁢          2      9        ⁢          t      .      The worst positioning error also results in a poor performance of the servo control unit 19 compensating the run-out error, and eventually results in a poor track following or a poor servo loop in the optical drive. Moreover, the poor track following or the poor servo loop is getting worst if the frequency-multiplier value is greater than 30.
Because the CAV of an optical drive is getting higher and higher, accordingly the servo control unit 19 needs more fixed positions to preciously control the optical pickup 11 for compensating the run-out error. For example, the blue-ray optical disc has to increase fixed positions to compensate the run-out error, and the positioning error may increase to a couple of clocks if the conventional method is used. Therefore, making the positioning error between the ideal frequency-multiplied FG signal and the actual frequency-multiplied FG signal within a specific range when the optical drive is operated at any CAV is the main purpose of the present invention.